Key Findings
The research indicates that CSRM can effectively track mechanical changes in the rear side of PV modules during weathering. In damp heat (DH) testing, the power degradation of the modules correlated with the initial cross-linking state of the encapsulants. While encapsulants initially showed an increase in cross-linking during the first 250 hours of DH testing, both the encapsulant and backsheet eventually degraded, leading to a decrease in the CSRM response. Furthermore, combined DH and ultraviolet (UV) weathering caused post-cross-linking within the first 250 hours, which leveled the cross-linking states and resulted in similar power degradation across samples.
Technical Details
The team utilized 200 mm × 200 mm mini-modules consisting of passivated emitter and rear contact (PERC) solar cells, ethylene vinyl acetate (EVA) encapsulants, and polyethylene/polyethylene terephthalate (PET) backsheets. To vary cross-linking degrees, lamination times at 155 C were adjusted. The CSRM process involved the following parameters:
- Test location heated to 89 C.
- A 4 mm steel tip pressed approximately 100 µm into the rear surface.
- Application of a force of less than 20 N.
- Measurement of force decline while maintaining constant indentation depth.
Modules underwent DH testing (85 C, 85% relative humidity) for up to 3,500 hours, or combined DH+UV testing (85 C, 60% relative humidity, 227 W/m2 irradiance) for up to 2,000 hours, with CSRM and electrical measurements taken every 250 hours.
Background & Context
CSRM is traditionally a non-destructive technique used in manufacturing quality control to assess encapsulant cross-linking after lamination. This study, conducted by scientists from Germany’s Anhalt University of Applied Sciences and Fraunhofer CSP, expanded its use to monitor the mechanical response of the rear side of modules during aging. Solvent tests and thermal analyses were used to decouple the degradation of the encapsulant from that of the backsheet.
Significance & Outlook
The study highlights that the backsheet undergoes chain scission and post-crystallization during degradation. The researchers conclude that overacceleration in testing must be avoided to ensure realistic reliability predictions for PV modules.
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